As one type of semiconductor substrate for producing a solar cell, polycrystalline silicon is widely used and its production amount rapidly increases every year. Polycrystalline silicon is generally formed either by filling silicon melt, which is melt by heating at high temperature, in a mold in which an inner surface of a quartz crucible, a dividable graphite crucible, or a quartz crucible installed inside a graphite crucible is coated with a mold release material by spray or by using a brush or a spatula followed by solidification, or by melting first a silicon material added in a mold followed by re-solidification.
The release layer prevents incorporation of impurities to a silicon ingot and adhesion of heat-melted silicon melt onto an inner surface of a crucible as a mold, and thus plays an important role of releasing a solidified silicon ingot from the mold. As for the mold release material, from the viewpoint of having generally high melting point and little contamination for a silicon ingot, high purity powder such as silicon nitride, silicon carbide, and silicon oxide, or their mixture powder is used. For increasing productivity of a silicon ingot, many researches and studies have been made regarding a method of forming a mold release material on a mold surface, a mold obtained by such treatment, or a method for producing a silicon ingot by using such a mold.
For example, Patent Literature 1 discloses a release layer of a mold for casting a silicon ingot in which silicon nitride powder obtained by thermal decomposition of silicon diimide and fine silica powder are used as a mold release material and the release layer is formed of a first layer on an inner side (i.e., mold side) of the release layer, which has high ratio of silicon nitride containing non-crystalline silica layer on surface of silicon nitride particles, and a second layer on an outer side (i.e., silicon ingot side) of the release layer, in which particles are strongly bound to each other as mixed with fine silica powder. It is also disclosed that, by means of the release layer, damages occurring on a solidified silicon ingot and a mold can be reduced during release. In addition, it is disclosed in Patent Literature 2 that, by using a release layer based on a mixture in which coarse melt silica sands are covered with silicon nitride powder, reaction between melt silicon and silica is inhibited so that the performance of a battery for solar power generation can be enhanced.